1. Field of the Invention
The present invention relates generally to the field of protein engineering. More particularly, it concerns methods for the screening of combinatorial libraries of polypeptides to allow isolation of enzymes having a desired catalytic activity and of ligand binding proteins, including antibodies and binding proteins having affinity for selected ligands
2. Description of Related Art
The isolation of proteins that either bind to ligands with high affinity and specificity or catalyze the enzymatic conversion of a reactant (substrate) into a desired product is a key process in biotechnology. Ligand-binding proteins and enzymes with a desired substrate specificity can be isolated from large libraries of mutants, provided that a suitable screening method is available. Small protein libraries composed of 103–105 distinct mutants can be screened by first growing each clone separately and then using a conventional assay for detecting clones that exhibit specific binding. For example, individual clones expressing different protein mutants can be grown in microtiter well plates or separate colonies on semisolid media such as agar plates. To detect binding the cells are lysed to release the proteins and the lysates are transferred to nylon filters, which are then probed using radiolabeled or fluorescently labeled ligands (DeWildt et al. 2000). However, even with robotic automation and digital image systems for detecting binding in high density arrays, it is not feasible to screen large libraries consisting of tens of millions or billions of clones. The screening of libraries of that size is required for the de novo isolation of enzymes or protein binders that have affinities in the nanomolar range.
The screening of very large protein libraries has been accomplished by a variety of techniques that rely on the display of proteins on the surface of viruses or cells (Ladner et al. 1993). The underlying premise of display technologies is that proteins engineered to be anchored on the external surface of biological particles (i.e., cells or viruses) are directly accessible for binding to ligands without the need for lysing the cells. Viruses or cells displaying proteins with affinity for a ligand can be isolated in a variety of ways including sequential adsorption/desorption form immobilized ligand, by magnetic separations or by flow cytometry (Ladner et al. 1993, U.S. Pat. No. 5,223,409, Ladner et al. 1998, U.S. Pat. No. 5,837,500, Georgiou et al. 1997, Shusta et al. 1999). The most widely used display technology for protein library screening applications is phage display. Phage display is a well-established and powerful technique for the discovery of proteins that bind to specific ligands and for the engineering of binding affinity and specificity (Rodi and Makowski, 1999). In phage display, a gene of interest is fused in-frame to phage genes encoding surface-exposed proteins, most commonly pIII. The gene fusions are translated into chimeric proteins in which the two domains fold independently. Phage displaying a protein with binding affinity for a ligand can be readily enriched by selective adsorption onto immobilized ligand, a process known as “panning”. The bound phage is desorbed from the surface, usually by acid elution, and amplified through infection of E. coli cells. Usually, 4–6 rounds of panning and amplification are sufficient to select for phage displaying specific polypeptides, even from very large libraries with diversities up to 1010 Several variations of phage display for the rapid enrichment of clones displaying tightly binding polypeptides have been developed (Duenas and Borrebaeck, 1994; Malmborg et al., 1996; Kjaer et al., 1998; Burioni et al, 1998; Levitan, 1998; Mutuberria et al., 1999; Johns et al., 2000).
One of the most significant applications of phage display technology has been the isolation of high affinity antibodies (Dall' Acqua and Carter, 1998; Hudson et al., 1998; Hoogenboom et al., 1998; Maynard and Georgiou, 2000). Very large and structurally diverse libraries of scFv or FAB fragments have been constructed and have been used successfully for the in vitro isolation of antibodies to a multitude of both synthetic and natural antigens (Griffiths et al., 1994; Vaughan et al., 1996; Sheets et al., 1998; Pini et al., 1998; de Haard et al., 1999; Knappik et al., 2000; Sblattero and Bradbury, 2000). Antibody fragments with improved affinity or specificity can be isolated from libraries in which a chosen antibody had been subjected to mutagenesis of either the CDRs or of the entire gene CDRs (Hawkins et al., 1992; Low et al., 1996; Thompson et al., 1996; Chowdhury and Pastan, 1999). Finally, the expression characteristics of scFv, notorious for their poor solubility, have also been improved by phage display of mutant libraries (Deng et al., 1994; Coia et al., 1997).
However, several spectacular successes notwithstanding, the screening of phage-displayed libraries can be complicated by a number of factors. First, phage display imposes minimal selection for proper expression in bacteria by virtue of the low expression levels of antibody fragment gene III fusion necessary to allow phage assembly and yet sustain cell growth (Krebber et al., 1996, 1997). As a result, the clones isolated after several rounds of panning are frequently difficult to produce on a preparative scale in E. coli. Second, although phage displayed proteins may bind a ligand, in some cases their un-fused soluble counterparts may not (Griep et al., 1999). Third, the isolation of ligand-binding proteins and more specifically antibodies having high binding affinities can be complicated by avidity effects by virtue of the need for gene III protein to be present at around 5 copies per virion to complete phage assembly. Even with systems that result in predominantly monovalent protein display, there is nearly always a small fraction of clones that contain multiple copies of the protein. Such clones bind to the immobilized surface more tightly and are enriched relative to monovalent phage with higher affinities (Deng et al., 1995; MacKenzie et al., 1996, 1998). Fourth, theoretical analysis aside (Levitan, 1998), panning is still a “black box” process in that the effects of experimental conditions, for example the stringency of washing steps to remove weakly or non-specifically bound phage, can only be determined by trial and error based on the final outcome of the experiment. Finally, even though pIII and to a lesser extent the other proteins of the phage coat are generally tolerant to the fusion of heterologous polypeptides, the need to be incorporated into the phage biogenesis process imposes biological constraints that can limit library diversity. Therefore, there is a great need in the art for techniques capable of overcoming these limitations.
Protein libraries have also been displayed on the surface of bacteria, fungi, or higher cells. Cell displayed libraries are typically screened by flow cytometry (Georgiou et al. 1997, Daugherty et al. 2000). However, just as in phage display, the protein has to be engineered for expression on the cell surface. This imposes several potential limitations. First of all, either the N-terminal or the C-terminal of the protein has to be fused to a vehicle for display. Thus, these technologies are not suitable where the N- or C-termini are essential for ligand binding. Second, the requirement for display of the protein on the surface of a cell imposes biological constraints that limit the diversity of the proteins and protein mutants that can be screened. Third, complex proteins consisting of several polypeptide chains cannot be readily displayed on the surface of bacteria, filamentous phages or yeast. As such, there is a great need in the art for technology which circumvents all the above limitations and provides an entirety novel means for the screening of very large polypeptide libraries.
At present, the isolation of novel enzymes from libraries of protein mutants is typically accomplished either through the use of a phenotypic selection or screening in either solid phase or microtiter well plates. Biological selections are based on complementation of auxotrophy or resistance to cytotoxic agents (e.g., antibiotics). Unfortunately, the utility of phenotypic selections is limited to the isolation of catalysts for reactions that are of direct biological relevance or can be indirectly linked to a selectable phenotype. Alternatively, each clone in a mutant population may be screened directly for enzymatic activity. For libraries expressed in microorganisms, screening can be performed on colonies growing on a solid substrate such as agar. Solid phase screening relies on substrates of an enzymatic reaction that give rise to a zone of clearance, a fluorescent product, or a strongly absorbing (chromogenic) product. The assay may detect the enzyme product directly or may be coupled to a second enzyme whose product can in turn be easily monitored. However, many assays cannot be implemented in a solid phase format. If that is the case then individual clones must be grown and assayed in microtiter wells. Such assays are significantly more time consuming than solid phase assays and severely limit the number of mutants that can be screened. However, when a small number of random mutants is screened, the probability of finding clones expressing an enzyme that can catalyze a desired biotransformation, especially when that biotransformation requires a complicated reaction, is severely affected.
In general, methods that will allow the screening of large libraries of enzyme mutants on the basis of kinetic parameters, i.e., on the basis of how much product is generated per unit time, are needed. Phage display technology may in principle be used as a tool for the isolation of useful enzymes from large libraries. However, harnessing phage display technology for the isolation of enzyme catalysts from libraries has thus far not proven practical (Olsen et al. 2000). For example there is no apparent way to physically link in a quantitative manner a phage particle displaying a certain enzyme clone with the outcome of multiple catalytic turnovers resulting in the accumulation of reaction product. Establishing such a linkage is essential for the screening of protein libraries on the basis of catalytic proficiency.